The problem with wood is poor design, not the material.
When I rebuilt my Scamp
with a 3/4" exterior plywood floor I covered both sides and the edges with fiberglass cloth and epoxy resin. The fiberglass interior is sealed completely and I can testify will hold water as during construction the rear window leaked and water was standing back there.
Polyester resin will allow water to penetrate over time and in my opinion is mostly an inexpensive route with many compromises.
Attention to detail and design to prevent water intrusion is the key, but in a semi-mass production setting the cost and time rules that out in most cases.
Mold does not need to have wood to form, just moisture and a source of food. Mold in my old Scamp
was mostly on and in the Ensolite insulation that forms the wall covering and took a lot of time and effort to eliminate.
I have found that keeping a dehumidifier going in the trailer has made a difference, in my case I keep the mini-split AC on year round.
If you do not keep up with the maintenance necessary to keep the water out then you will have problems.
Sealing the penetrations well and proper design should allow the use of one of the most useful building materials with few problems.
While many of these small trailers have problems with rotten floors it is due to neglect and poor design. Spraying polyester resin on the bottom of the floor does nothing to protect from the most likely cause of floor rot, leaking windows
and openings for heaters, refrigerators, etc.
My trailer was made to eliminate most of these intrusions other than windows
. This is the reason for the compressor refrigerator
and heat pump, not to mention the tankless hot water heater on the tongue.
There are openings for the potable water reel and the black water reel, but again these compartments are sealed with epoxy fiberglass as is the storage compartment on the street side.
These small fiberglass trailers are already more expensive than others due to construction and eliminating the wood, while is an intriguing idea, would price them out of most budgets. The cost would be similar the the Airstream NEST which is breathtakingly expensive for the size.
If one really wanted to build something like this then a foam inner core with epoxy fiberglass would do the trick, but the foam can also absorb water and also delaminate as seen in many "fiberglass" "stick" trailers.
To each his own and good luck with your search.
By the way even with all of these real problems these molded fiberglass trailers seem to last a very long time, mine ,requiring a rebuild, is 35 years old.